A typical type of image forming apparatus includes a plurality of image forming units (forming devices) that are arranged along a sheet conveying belt so as to transfer toner images in different colors one by one to a sheet conveyed on the belt. In order to ensure the quality of the formed images, an art generally referred to as registration and the like has been adopted to this type of image forming apparatus. With the art, the image forming units form a pattern including a plurality of marks on the surface of the belt; an optical sensor measures a position of each mark to detect the deviation amount of the image forming position with respect to each color; then, on the basis of the measurement result, the deviation amount of the image forming position with respect to each color is corrected.
Such an art, generally, is addressed to correct a steady positional deviation amount due to a positional deviation of an image forming unit member (a photosensitive drum, an optical members of an exposure unit) and the like. On the other hand, there is also an art addressed to correct a varying positional deviation amount, which has a specific cycle, due to eccentricity of a photosensitive drum and/or a roller that support(s) the belt, irregularity in pitch of a gear whereby these members are rotationally driven, and the like. With this art, another pattern (a pattern other than the pattern for the steady positional deviation detection) is formed on the belt, the cyclic positional deviation amount of the image forming position is detected, and, on the basis of a result of the detection, the image forming position is corrected.
By frequently performing such positional deviation detection, the quality of the formed image can be maintained. However, more frequent detection causes more consumption of coloring agent and more time for the user to wait. Therefore, there is a need for an image forming apparatus that can perform the positional deviation detection at suitable timings.